The present invention relates to a node, and specifically to a method for notifying a defect on a multipoint logical path and switching a logical path in a transfer protocol that constructs a logical path, which is typified by MPLS etc., among plural nodes being connected to a ring network, and a node having such functions.
In response to wider bands of access networks in recent years, a service of distributing broadcast services, as that of mass media such as current television broadcasting, to users in a multicast scheme has begun to be offered. In such broadcast distribution services, a form in which distribution equipment installed in a WAN (Wide Area Network) side to the users distributes a data stream aimed at the users can be postulated. In the broadcast services and the like, service interruption caused by a defect in a transmission path is not allowed. Therefore, in the WAN, means for detecting a defect in the multipoint logical path that connects one transmitting end point with plural receiving end point and means for switching a path become essential. The ring network employs a network topology frequently used for facilities of the WAN (Wide Area Network), the MAN (Metro Area Network), etc. from viewpoints of the ease of path management, defect point detection, etc. FIG. 1 is a diagram for illustrating a general ring network. Four nodes, from a node 101-1 to a node 101-4, are connected mutually by two transmission paths. In the ring network, in order to provide services without interruption even in the occurrence of a defect, two routes of a working ring 102 and a backup ring 103 are provided. In the WAN, the MAN, etc. provided by ISP's and carriers, since high reliability is required, maintenance/management functions, such as monitoring a state of the transmission path, detecting a defection state immediately, and bypassing the defect point, are mandatory.
Therefore, a method for detecting a defect and notifying it is specified in “point-to-point ATM logical path” by ITU-T Recommendation I.610, “B ISDN operation and maintenance principles and functions” (nonpatent document 1) and in “point-to-point MPLS logical path” by ITU-T Recommendation Y.1711, “Requirements for Operation & Maintenance functionality for MPLS networks” (nonpatent document 2), respectively. In the nonpatent document 1 and the nonpatent document 2, continuity of the logical path is checked using a healthcheck frame of a path. Moreover, when a defect is detected on the logical path, the defect is notified to a data receiving end node by a forward defect indication frame. On the other hand, the data receiving end node notifies the defect to a data transmitting end node by a backward defect indication frame. In this occasion, since upward/downward bi-directional logical paths are constructed in the case of the point-to-point logical path, the forward defect indication frame is forwarded to the receiving end node using a path in which the defect occurs, whereas the backward defect indication frame is forwarded to the transmitting end node using a counter path. Hereinafter, the healthcheck frame, the forward defect indication frame, and the backward defect indication frame are generically designated as OAM (Operation, administration and maintenance) frames.
In addition, for example, IEEE802.17 (RPR: Resilient Packet Ring) (nonpatent document 3) specifies means for bypassing a defect point in the occurrence of a defect. The nonpatent document 3 describes a procedure of, when a defect is detected in a physical layer, performing the defect point bypassing as follows.    1. A defect detection position is forwarded to nodes that constitute the ring.    2. When defect information is received, each node calculates a route again.    3. A route that does not pass the defect occurrence point is configured.